Substitution‐Controlled Excited State Processes in Heteroleptic Copper(I) Photosensitizers Used in Hydrogen Evolving Systems

Abstract

Four different heteroleptic [Cu(N^N)(P^P)]PF6 complexes, which combine classical bidentate diimine ligands and sterically demanding diphosphine ligands, are studied by a combination of ultrafast time-resolved spectroscopy and quantum chemical calculations. The light-induced excited state processes, accompanied by a structural change, are discussed with respect to the application of these complexes as a new class of noble-metal-free photosensitizers in proton reducing systems. In particular, the influence of different substituents in the ligand backbone on the photophysical properties is highlighted.

Heteroleptic Copper Photosensitizers: Why an Extended π‐System Does Not Automatically Lead to Enhanced Hydrogen Production

“…The heteroleptic complexes 1 – 3 strongly absorb light in the UV region, but also in the visible region up to 450 nm (Figure A), which results from ligand‐centered and MLCT transitions, respectively. The absorption band above 355 nm can be assigned to transitions from the d orbitals of the copper center to the π* orbital of the diimine ligand and not to the Xantphos ligand. As a consequence, the extinction coefficients in the range of the MLCT transitions of 1 and 2 are lower than that of 4 (Table ).…”

“…It has been found that the ligand environment and the substituents on these ligands allow for fine tuning of the photophysical and electrochemical properties of the resulting photosensitizers. For instance, introduction of alkyl substituents at the 2,9‐position of 1,10‐phenanthroline (phen) leads to an increased excited state lifetime owing to sterical hampering of structural relaxation by Jahn–Teller distortion, which decelerates undesired exciplex quenching . However, these heteroleptic Cu I complexes still suffer from limited absorption in the visible region and lower metal‐to‐ligand charge‐transfer (MLCT) extinction coefficients, especially when compared with advanced Ru II and Ir III photosensitizers …”

Effective Quenching and Excited-State Relaxation of a Cu(I) Photosensitizer Addressed by Time-Resolved Spectroscopy and TDDFT Calculations

“…The spectrum is identical to the one observed in femtosecond absorption experiments for delay times longer than 20 ps and which was assigned to the relaxed 3 MLCT state of the CuPS. [35] The present experiments show that the signal decreases with an exponential decay time of 280 ns to zero. ⋅ exp(−t/ . )…”

Light to Hydrogen: Photocatalytic Hydrogen Generation from Water with Molecularly-Defined Iron Complexes

“…In the case of the Cu-PS D, ISC takes 7 ps and is preceded by a flattening of the complex structure within the first picosecond [156]. As in the case of the Ir-PS C, the long lifetime of the resulting 3 MLCT is a crucial factor for the performance as a sensitizer in photocatalysis.…”

“…Taking this into consideration, significantly improved quantum yields of up to 48% at a wavelength of 415 nm and 41% at 440 nm, respectively, were achieved (Figure 2 [154] and at the same time favor the tetrahedral (Td) geometry. Thus, in the MLCT exited state, the flattening to the preferred square planar geometry is significantly reduced [155,156]. A number of molecularly-defined Cu-PS has been synthesized based on various combinations of bidentate phosphines and amines.…”